Mechanical vibration may be created within an aircraft structure due to rotational unbalances in its engine(s). For example, on fuselage-mounted engines, the rotational unbalance(s) may cause vibration transmission into the pylon or other like connecting structure thereby vibrating the fuselage. If the fuselage vibration is well coupled to the acoustic space of the aircraft cabin, a predominantly tonal acoustic noise (generally characterized as a low frequency drone) may be generated. In particular, this drone is related to the rotation of the fan stage (at an N1 frequency) and/or of the compressor stage (at an N2 frequency). In aircraft with aft-fuselage-mounted engines, such as the McDonnell Douglas DC-9, any rotational unbalance of the engines may result in such low-frequency cabin noise. This is particularly noticeable in the cabin's aft-most section. Elimination or reduction of these N1 and/or N2 tones can dramatically reduce the annoyance experienced by passengers.
Within the prior art, various means are known for attenuating such noise and vibration. These include passive blankets, Active Isolation Control (AIC), Active Noise Control (ANC), Active Structural Control (ASC), passive Tuned Vibration Absorbers (TVAs), and Adaptive TVAs (ATVAs). Passive blankets are generally effective for attenuating high-frequency noise, but are generally ineffective at attenuating lower-frequency noise. Where a higher level of noise attenuation is desired, Active Isolation Control (AIC) systems may be utilized.
AIC systems include active mountings which accommodate engine loads/motions and include an actively-driven element to provide active (at the frequency of the vibration) control forces. These active forces prevent vibration transmission from, for instance, the aircraft's engines into the pylon structure. The resultant effect is a reduction of cabin noise.
AIC systems include the feedforward-type, in which reference signals provide reference signals representative of the engine(s) vibration(s). A plurality of distributed sensors, such as microphones, provide signals representative of residual noise at various cabin locations. These reference and error signals are processed by a digital controller to generate oscillatory anti-vibration drive signals to the actively-driven elements in the mounts. These anti-vibration signals are of the appropriate amplitude, phase, and frequency to control vibration transmission from the engine to the pylon, thereby minimizing, to the extent possible, unwanted interior noise. U.S. Pat. No. 5,551,650 entitled "Active Mounts for Aircraft Engines" describes one such AIC system. Disadvantages of AIC systems include space requirements for housing the active element, difficulty of accomplishing the appropriate actuation directions for vibration attenuation, and relatively high cost.
Active Noise Control (ANC) systems may be used, for example, on turboprop aircraft, and include a plurality of loudspeakers, strategically located within the aircraft cabin. These loudspeakers are driven responsive to reference signals representative of engine or propeller information and error signals from error sensors dispersed in the cabin. The oscillatory drive signals to the loudspeakers are generally controlled via a digital controller according to a known control algorithm, such as the Filtered-x Least Mean Square (LMS) algorithm. Co-pending U.S. patent application Ser. No. 08/553,227 to Billoud entitled "Active Noise Control System For Closed Spaces Such As Aircraft Cabins", describes one such ANC system. ANC systems have the disadvantage that they do not generally address mechanical vibration problems, may be difficult to retrofit into existing aircraft due to potentially significant interior modifications, and are somewhat costly. Furthermore, as the frequency of noise increases, large numbers of error sensors and speakers are required to achieve sufficient global noise attenuation.
Active Structural Control (ASC) systems, may solve the problem of needing a large number of error sensors by attacking the fuselage's vibrational modes directly. For example, by attaching a vibrating device, such as an actuator or shaker, to the interior surface of the fuselage as described in U.S. Pat. No. 4,715,559 to Fuller, global attenuation may be achieved. However, the modifications necessary to retrofit Active Vibration Absorbers (AVAs) in this manner may be prohibitive, as the interior trim may have to be removed and structural modifications made to the stiffening frame members. Therefore, prior art ASC systems are necessarily difficult to retrofit.
Further descriptions of AVAs and active mounts can be found in WO 96/12121 entitled "Active Systems and Devices Including Active Vibration Absorbers (AVAs)". As should be apparent from the foregoing, Active Noise and Vibration Control (ANVC) systems are attractive but somewhat complex and, thus, tend to be more expensive than simpler attenuation systems. Passive and adaptive systems may offer these simpler, less-expensive alternatives.
Passive Tuned Vibration Absorbers (TVAs) are effective at attenuating low-frequency vibration, but are limited in range and effectiveness. Passive TVAs include a flexibly-suspended tuned mass which is tuned by adjusting the stiffness of its flexible suspension or the mass of the suspended tuned mass, such that the device exhibits a stationary resonant frequency (fn). Vibration of the tuned mass absorbs vibration of a vibrating structure at its attachment point. Problematically, TVAs may be ineffective if the engine speed changes, such that the TVA's resonant frequency no longer coincides with the disturbance frequency. U.S. Pat. No. 3,490,556 to Bennett, Jr. et al. entitled: "Aircraft Noise Reduction System With Tuned Vibration Absorbers" describes a passive vibration absorber for use on the engine-mounting pylon of an aircraft for absorbing vibration at specific N1 and N2 frequencies.
When vibration cancellation over a wider range of frequencies is required, various Adaptive TVAs (hereinafter referred to as ATVAs) may be employed. For example, U.S. Pat. No. 3,487,888 to Adams et al., entitled "Cabin Engine Sound Suppresser" teaches an ATVA whose resonant frequency (fn) can be adaptively adjusted by changing an effective length of a beam. Other examples of adaptive vibration absorbers can be found in U.S. Pat. Nos. 5,564,537, 5,236,186, 5,197,692, 5,072,801, 3,767,181, 3,430,902. A good example of 90.degree. phase difference control for an ATVA may be found in U.S. Pat. No. 3,483,951 to Bonesho et al.
In ATVAs including a vibratory mass moving relative to a base, the base motion and the mass motion are usually measured by sensors (ex. acceleration sensors) which produce appropriate vibration signals. Generally, one vibration signal is generated for each motion, i.e., base and mass motion. The signals are then used to generate a control signal which can be used to slowly adjust the relative phase difference between the two signals. When the signals are in quadrature (90.degree. phase difference), good vibration absorption is achieved.
Notably, the frequency range of attenuation may be greatly increased with ATVAs over passive TVAs. Also, the controls needed for ATVAs are relatively simple in comparison to fully-active systems (ex. ANC, ASC, AIC systems). This is because the system adjustments are much slower than the frequency of vibration. The term "Adaptive" as used herein refers to a slow adjustment to a physical parameter (stiffness or mass) of the system whereby the adjustments are made at a speed much slower that the vibrational frequency. By way of example, and not to be considered limiting, 130 Hz vibrations may be attenuated by the TVA, yet the change to the stiffness or mass parameter may occur at only a fraction of that frequency, for example, at 0.2-1.0 Hz. An example of an ATVA may be found in Lawrence Andrew Mianzo's Mechanical Engineering Thesis entitled "An Adaptable Vibration Absorber To Minimize Steady State And Transient Vibrations--An Anaytical and Experimental Study" from Penn State University dated August 1992. As best seen in FIG. 3.3 of that thesis, the ATVA includes a upper and lower leaf springs, first and second absorber masses attached at the outer ends of the leaf springs, and a lead screw nut which threadedly receives a lead screw attached to a stepper motor. The stepper motor is stationarily mounted relative to the lower leaf. Rotation of the stepper motor rotates the lead screw which spreads the leaf springs thereby effecting a change in the stiffness of the leafs. This adaptively changes the resonant frequencies of the ATVA. The Mianzo device, although sufficient for axial vibration suppression, is inadequate for tangential and radial absorption. Further, the Mianzo device includes 2 masses, thus is susceptible to having two resonances. Moreover, the Mianzo device requires either a very large torque motor for spreading high stiffness leaf springs, resulting in a heavy device, or if a small motor is used, the ATVA is only capable of exhibiting low resonant frequencies because of the necessarily low stiffness leaf springs.
Therefore, a need exists for tuned vibration absorber and systems with good vibration response, which are compact, light, have low power requirements and adaptive TVA devices which are readily tunable to specific resonance frequencies.